The present invention relates to a method of joining a rubber magnet to a yoke used in an electrical apparatus such as a brushless motor.
FIGS. 5A and 5B are used for explaining a conventional method of manufacturing a magnet rotor 101 of the brushless motor of an outer rotor type used as a fan motor. FIG. 5A is a top plan view of the magnet rotor 101 seen from an axial direction of a rotating shaft 105, while FIG. 5B shows a vertical section of the magnet rotor 101. FIG. 6 shows a vertical section used for explaining a manufacturing step of the magnet rotor 101.
The magnet rotor 101 is constituted from a cup-shaped yoke 103, the rotating shaft 105, and a bracket for securing the rotating shaft to the yoke 103. The cup-shaped yoke 103 is constituted from a cylindrical peripheral wall section 111 and a disc-shaped bottom wall section 112 integral with the peripheral wall section 111 so as to close an end of the peripheral wall section 111. A through hole 113 is formed at a center of the bottom wall section 112. The bracket 107 is fitted into the through hole 113 for securing the rotating shaft 105 to the yoke 103. The bracket 107 is constituted from a bracket body 108 having at its center a through hole into which the end of the rotating shaft 105 is fitted and a ring shaped flange section 109 integral with the end of the bracket body 108. An outer peripheral surface 119 of a cylindrical rubber magnet 117 is joined to an inner peripheral surface 114 of the peripheral wall section 111 of the yoke 103. The cylindrical rubber magnet 117 is formed by rolling a plate-like rubber magnet into a cylindrical shape.
The cylindrical rubber magnet 117 is joined to the inner peripheral surface 114 of the peripheral wall section 111 of the yoke 103 in a manner as described below. First, an adhesive 115 is applied to the inner peripheral surface 114 of the peripheral wall section 111 of the yoke 103. The adhesive 115 is continuously applied to a middle region of the inner peripheral surface 114 in a peripheral direction. Next, the long, thin, and plate-like rubber magnet is rolled into the cylindrical shape, thereby forming the cylindrical rubber magnet 117. Then, the cylindrical rubber magnet 117 is inserted into a predetermined position by sliding its outer peripheral surface 119 over the inner peripheral surface 114 of the peripheral wall section 111 of the yoke 103. The adhesive 115 is then cured, thereby completing the manufacturing of the magnet rotor.
Japanese, Patent Application Laid-Open Publication No. 146101/1993 discloses in its FIG. 2 a conventional technique by which a sheet-like rubber magnet is press fitted into a cylindrical frame and adhered to the frame by the adhesive.
In a conventional joining method, however, part of the adhesive 115 applied to the inner peripheral surface 114 of the yoke 103 tends to be pushed out of a lower end surface 121 of the cylindrical rubber magnet 117 to the bottom wall section 112, as shown in FIG. 6, when the cylindrical rubber magnet 117 is inserted into the yoke 103. For this reason, a necessary and sufficient amount of the adhesive fails to uniformly get into a gap between the inner peripheral surface 114 of the yoke 103 and the outer peripheral surface 119 of the cylindrical rubber magnet 117. If such a situation occurs, it becomes more likely that the rubber magnet will be partially separated from a joint surface between the yoke 103 and the cylindrical rubber magnet 117. If such a separation occurs, part of the cylindrical rubber magnet 117 is displaced inwardly in a radial direction of the rotor, and a displaced part of the cylindrical, rubber magnet 117 might come into contact with pole surfaces of a stator (not shown).
Especially when a high-performance rubber magnet such as an Nd—Fe—B based rubber magnet is employed so as to enhance performance of a motor, it becomes more likely that an adhesion layer will come off due to a strong magnetic force between the cylindrical rubber magnet 117 and the stator (not shown). Alternatively, the stronger the magnetic-force becomes, the more likely it becomes that the part of the separated cylindrical rubber magnet 117 will be displaced toward the stator and come into contact with poles of the stator, thereby causing faulty rotation of the motor due to a magnetic attraction force that acts between the separated part of the cylindrical rubber magnet 117 and the stator.